2025ApJ...991..197X

We present new observational evidence supporting the hypothesis that SQ-A, a starburst in the intragroup medium (IGrM) of Stephan's Quintet (SQ), is triggered by a high-speed collision between two gas systems, one associated with the IGrM (v ∼ 6900 km s^–1) and another with the intruder galaxy NGC 7318b (v ∼ 6000 km s^–1). The new CO(2-1) data set from the Atacama Large Millimeter/submillimeter Array has angular resolutions between $0\mathop{.}\limits^{^{\prime\prime} }2$ and $7\mathop{.}\limits^{^{\prime\prime} }0$ and the new Very Large Array H I data cube has an angular resolution of $6\mathop{.}\limits^{^{\prime\prime} }6\times 7\mathop{.}\limits^{^{\prime\prime} }9$. The CO maps show that the two gas systems are bridged by another system with an intermediate velocity of ∼6600 km s^–1, whereas the H I data show that the component of v ∼ 6600 km s^–1 fits well into a gap in the more extended v ∼ 6000 km s^–1 component, albeit with a displacement of ∼5 kpc. Both the bridge and the complementary distributions between different gas systems are common features of starbursts triggered by cloud-cloud collision. An analysis of clumps (sizes of 100-200 pc) reveals very diversified star formation (SF) activity in clumps belonging to different kinematic systems, with the molecular gas depletion time of the v ∼ 6900 km s^–1 clumps being more than 10 times longer than that of the v ∼ 6600 km s^–1 clumps. The results are consistent with a scenario in which the enhanced SF activity (and the starburst) in the system of v ∼ 6600 km s^–1 is due to gas compression generated in cloud-cloud collisions, whereas the suppression of SF in the v ∼ 6900 km s^–1 system is due to vortices (i.e. gas rotation) generated in more complex collisions involving dense clouds and diffuse intercloud gas associated with high-speed shocks.